Degradation biosensing performance of polymer blend carbon nanotubes (CNTs) nanocomposites

Sensors and Actuators A 295 (2019) 113–124

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Degradation biosensing performance of polymer blend carbon nanotubes (CNTs) nanocomposites Yasser Zare a , Hamid Garmabi a , Kyong Yop Rhee b,∗ a b

Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran Department of Mechanical Engineering, College of Engineering, Kyung Hee University, Yongin 446-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 1 March 2019 Received in revised form 19 May 2019 Accepted 23 May 2019 Available online 31 May 2019 Keywords: Polymer nanocomposite Hydrolytic degradation Sensing behavior Electrical conductivity

a b s t r a c t This paper describes the hydrolytic degradation and sensing behavior of poly (lactic acid) (PLA)/poly (ethylene oxide) (PEO)/carbon nanotubes (CNTs) nanocomposites in neutral phosphate-buffered saline (PBS) solution. Degradation sensing is performed based on the variations of electrical conductivity due to the degradation of PLA and PEO phases. The molecular weights of polymers, the morphologies of polymer blends and nanocomposites, the chemical bonds and the crystalline structures of polymers are analyzed before and during the degradation process. CNTs promote the degradation of PLA/PEO blends and the conductivity of all samples improves with increased degradation time. The samples with high PEO and CNTs concentrations exhibit a considerable sensing, because they show a large variation in conductivity during degradation. A matrix-droplet structure forms in the samples before and after degradation, where PEO particles are dispersed in a continuous PLA phase. Both amorphous and crystalline regions of PEO experience the hydrolytic degradation, but CNTs largely control the crystallinity of PLA suggesting the localization of CNTs in the PLA phase. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The environmental, biomedical and agricultural applications of biodegradable polymers have inspired extensive research in recent years [1–10]. Among the biodegradable polymers, polyesters play a main role due to their hydrolysable ester bonds, which degrade in a humid environment. Poly (lactic acid) (PLA) is a polyester with high strength and good biocompatibility that is commonly used in biomedical applications [11–13]. Numerous hydrolytic degradation studies have been performed on PLA to simulate its degradation in the human body and natural locations such as soil or compost [14–17]. These reports showed that PLA is hydrolyzed and produces water-soluble oligomers. Many parameters such as the fractions of amorphous and crystalline regions, the shape of the specimen, the chemical structure of polymer as well as the conditions (pH and temperature of the medium) influence the rate of degradation [18,19]. The unfavorable degradation rate of PLA limits its applications, and many researchers have tried to control the degradation rate (typically seeking to accelerate it) and develop PLA applications in biomedical or ecological areas. The degradation can be governed

∗ Corresponding author. E-mail address: [email protected] (K.Y. Rhee). https://doi.org/10.1016/j.sna.2019.05.040 0924-4247/© 2019 Elsevier B.V. All rights reserved.

by blending PLA with additives, plasticizers and inorganic fillers [20,21]. Previous studies reported the degradation behavior of PLA with the addition of many nanoparticles such as montmorillonite clay, titanium dioxide, silica and polyhedral oligomeric silsesquioxanes (POSS) [14,18,22,23]. Among all nanofillers, carbon nanotubes (CNTs) have gained much interest due to their high modulus (1000 GPa) and excellent electrical conductivity (105 S/m) [11,24–39]. CNTs change the crystallinity of PLA, and thus the nanocomposite can exhibit high hydrolytic degradation ability [20,40]. The diffusion of solvent into amorphous regions of PLA eliminates the chain entanglement and accelerates the degradation. Some authors recently studied the degradation behavior of PLA/CNTs nanocomposites. Chen et al. [20] found that poly (Llactide)/CNTs displays high hydrolytic degradation ability. They indicated that the hydrolytic degradation greatly depends on the CNTs content. Also, Mai et al. [41] prepared the conductive PLA/CNTs nanocomposites to develop an intelligent system for sensing degradation. They successfully correlated the variation of electrical resistivity with degradation level of PLA. In addition, they demonstrated the outstanding degradation sensing of nanocomposites at CNTs concentrations around the percolation threshold because the CNTs networks form at percolation threshold and the conductivity of nanocomposites drastically increases at this point [42–46]. In contrast to many other stimuli, biodegradation resulted

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in a reduction in resistivity due to an improvement in the CNTs network density after partial removal of the amorphous regions of polymer matrix [41]. The hydrolytic degradation of PLA blends containing nanoparticles has not been studied in the literature. The morphology of the blend and the localization of nanoparticles in one phase can affect the properties such as degradation rate [47]. Moreover, highly conductive CNTs improve the conductivity of polymer blends, allowing us to study the degradation sensing of nanocomposite as a function of blend structure and the characteristics of the CNTs network. The addition of a water-soluble polymer like poly (ethylene oxide) (PEO) into the PLA/CNTs nanocomposite may affect the degradation behavior of PLA. Nakafuku and Sakoda [48] and Agari et al. [49] reported a partial miscibility between PLA and PEO phases. It means that only some fractions of PLA and PEO produce a homogenous blend. In other words, the blending of PLA and PEO may produce a homogenous or inhomogeneous product. Therefore, the properties of both polymers and the presence of CNTs in one phase largely control the degradation rate and sensing behavior of the PLA/PEO nanocomposite. The degradation rate is an important parameter for biomedical applications. Moreover, a high degradation rate without the aid of enzymes is necessary for some applications. In this paper, different compositions of PLA/PEO/CNTs nanocomposites are prepared by solution mixing in chloroform. The hydrolytic degradation and sensing behavior of the prepared samples in phosphate-buffered saline (PBS) solution are studied for 4 weeks. The degradation sensing is monitored by the change of the electrical conductivity due to the degradation of PLA and PEO phases, which affects the characteristics of CNTs networks in nanocomposites. The morphology, crystallinity and the chemical bonds of samples before and after degradation process are analyzed. These products are conductive and sensitive to degradation stimuli, which can be used in various biomedical applications such as prosthetic devices, biosensors, actuators and drug delivery. 2. Experimental 2.1. Materials PLA (biopolymer, ME346310), PEO with viscosity average molecular weight (Mv ) of 200,000 g/mol) and phosphate-buffered saline (PBS) powder were purchased from Sigma-Aldrich Chemical Co. Multi-walled carbon nanotubes (MWCNTs) (CM-95) were provided by Hanhwa Nanotech CO., Korea. The lengths and diameters of the MWCNTs were about 10 ␮m and 20 nm, respectively. Chloroform was also purchased from Shindo Co., Korea. The chemicals were used as received. 2.2. Sample preparation PLA/PEO/CNTs nanocomposites were prepared through solution mixing using chloroform as a solvent for both PLA and PEO. The desired amount of CNTs was dissolved in chloroform at a concentration of 1 mg/ml by stirring and sonication (at 300 W) for 8 h to make a homogeneous dispersion. Both PLA and PEO were completely dissolved in chloroform at a concentration of 100 mg/ml. The polymer solution was subsequently mixed with a CNTs suspension and stirred for 5 h to obtain a homogeneous solution. After sonication for 1 h, the solvent was evaporated at room temperature and then the PLA/PEO/CNTs film was prepared. Finally, the film was dried at 45 ◦ C under vacuum for 12 h to remove the solvent. The concentrations of PLA in the blends and nanocomposites were 60, 75 and 90 wt%. Also, CNTs contents of 1 and 2 wt% were chosen. The samples were designated as PLAx/PEOy/CNTz, where x, y and z show the weight fraction of PLA, PEO and CNT, respectively.

2.3. Hydrolytic degradation Samples with a diameter of 20 cm and a thickness of 1 mm were dried and immersed in PBS solution (pH = 7) at room temperature. After immersion for certain periods of time, the samples were removed from solution, washed several times with distilled water and dried in an oven until they reached a constant weight. The samples were carefully weighed to evaluate the fraction of hydrolytic degradation. The degradation fraction was calculated by: Degradatio n fraction = 1 −

M M0

(1)

where M0 and M represent the initial weight (g) of the sample before degradation and the residual weight (g) after degradation for a certain time, respectively. The degradation rate (% per week) was also defined as: Degradatio n rate =

M0 − M 100 NM 0

(2)

where “N” denotes the degradation time in weeks. 2.4. Characterization The number average molecular weight (Mn ) was determined by gel permeation chromatography (GPC). Measurements were performed in THF with a Waters 717 using a polystyrene standard. Then, 100 ml of a polymer solution with a concentration of 1 mg/ml was injected at 1 ml/min. A 4-point Mitsubishi LORESTA GP was used to measure the conductivity of all films. A silver paste coating was applied to provide good contact with the electrodes of the electrometer. Four measurements on different spots of each sample were performed. An average value and standard deviation in the mean were reported. The morphologies of the samples were studied on gold-coated samples using scanning electron microscopy (SEM) (XL30S) to investigate the structure of the blend and the dispersion of CNTs in the matrix at an accelerating voltage of 10 kV. The Fourier transform infrared spectroscopy with attenuated total reflectance analysis (FTIR-ATR) was carried out using a PerkinElmer Multiscope FTIR spectrometer at room temperature in the range of 400-4000 cm−1 . The height ratios of main peaks were determined using Sabnis and Block expression [50] as: Peak height ratio =

Hpeak X HRef Peak

(3)

where HPeak X and HRef Peak denote the heights of study peak and reference peak, respectively. The peak at 1450 cm−1 allocated to the CH3 deformation is considered as reference peak, which is proper for PLA [51]. HPeak is obtained by: HPeak = Log10

AC BC

(4)

where AC is the height of the base line and BC is the absolute height of the study peak. The crystalline structures of polymers before and after degradation were investigated by wide angle X-ray diffraction (WAXD, DX-1000). The scanning angle range was from 2◦ to 30◦ at 35 kV and 20 mA. 3. Results and discussion 3.1. Degradation behavior The exact weights of samples before and during degradation were measured and substituted into Eq. 1 to calculate the degradation fraction. Fig. 1 (a, b, c) depicts the variations of degradation

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Fig. 1. The degradation fraction and rate of samples containing (a, d) 60, (b, e) 75 and (c, f) 90 wt % PLA in PBS solution as a function of time.

fraction as a function of time (weeks). It is clear that the samples quickly degrade during the first week, but the degradation fraction insignificantly changes after 1 week. During the first week, PEO rapidly degrades in PBS solution, while PLA degradation is negligible. After 1 week, the degradation of PEO becomes slow and PLA prevents the further degradation of PEO. Moreover, it is obvious that CNTs increase the degradation of samples during the degradation period. Therefore, CNTs promote the degradation of the PLA/PEO blend because CNTs play a catalytic role in the degradation process [41]. The PLA60/PEO40 blend and its nanocomposites show the high degradation fraction, while the samples consisting of 90 wt% PLA display the poor degradation. The slowest and the most rapid degradation are observed in PLA60/PEO40/CNT2 and PLA90/PEO10 samples, respectively. Thus, the concentrations of PEO and CNTs have the most influence on the extent of degradation in the samples. The degradation process of polymers such as PLA can be summarized as the diffusion of PBS solution into the polymer, a reduction in polymer molecular weight accompanied by the

formation of oligomers and monomers and finally the dissolution of oligomers and monomers in solution [14]. It is clear that the rate of each stage influences the total degradation rate of the polymer. The degradation rate is estimated by replacing the exact weights of samples before and after degradation at a certain time into Eq. 2. Fig. 1 (d, e, f) illustrates the degradation rate of samples in PBS solution at different times. The degradation rate decreases as the degradation time increases. However, the degradation rate of the blend and nanocomposites becomes relatively constant after 4 weeks. A high degradation rate is observed in the samples containing a high PEO fraction. As mentioned, PEO mainly degrades at the early stages of degradation, but PLA prevents the additional degradation of PEO in the later weeks. In fact, the PEO phase accelerates the hydrolytic degradation of samples, but the degradation rate decreases after the degradation of PEO phase. Furthermore, the degradation rate increases by addition of CNTs to the blend. The maximum difference between the degradation rate of the blend

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Fig. 2. The variation of polymer molecular weights in a) PLA75/PEO25 and b) PLA75/PEO25/CNT2 samples during the hydrolytic degradation.

and nanocomposites is observed in the samples containing 75 wt% PLA. However, CNTs in the samples containing 60 wt% PLA insignificantly change the degradation rate. The variation of degradation behavior in the samples depends on the morphology and localization of CNTs in the polymer blends. Since the contents of PEO and CNTs in the samples affect the sample structure, different degradation rates are expected. Generally, PLA60/PEO40/CNT2 and PLA90/PEO10 samples reveal the highest and the smallest degradation rates, respectively. Fig. 2 displays the molecular weights of PLA and PEO in PLA75/PEO25 and PLA75/PEO25/CNT2 samples before and during the degradation period. The molecular weight of PEO mainly decreases during degradation, but PLA shows a negligible variation in the molecular weight at different times. Accordingly, the size of PEO chains mainly decreases during degradation, while the length of PLA chains insignificantly decreases at the same time. These remarks indicate the extensive degradation of PEO in the initial 2 weeks. However, the molecular weight of PEO slightly changes after 2 weeks demonstrating the insignificant degradation of PEO at this period. Additionally, CNTs mainly reduce the PEO molecular weight over time, while the molecular weight of PLA shows negligible change. It is obvious that the CNTs largely degrade the PEO chains in the first week of hydrolytic degradation confirming the catalytic role of CNTs in the degradation process. These results agree with the reported data in the literature for hydrolytic degradation of PLA/TiO2 nanocomposites [14]. 3.2. Sensing properties Fig. 3 shows the conductivity of nanocomposites as a function of degradation time. The conductivity of all samples increases with increased degradation time. Thus, degradation desirably affects the conductivity of samples because it increases the effectiveness of CNTs networks in the samples. In fact, the degradation reduces the concentration of insulated polymer phases in the samples, increasing the concentration and efficiency of CNTs. Thus, it is logical to obtain a higher conductivity at longer degradation times. Other researchers also reported an improvement in conductivity by increasing degradation time [41]. In addition, it is clear that the high content of CNTs increases the conductivity in the same blend because more CNTs facilitate electron transfer in the nanocomposites. However, the nanocomposites containing 60 wt% PLA show the highest conductivity among the prepared samples due to the localization of CNTs in the PLA phase, which produces the most efficient networks for charge transfer. On the other hand, the dispersion of CNTs in the major PLA phase of PLA90/PEO10/CNT1 decreases the conductivity at various degradation times.

The improvement of conductivity due to degradation of the polymer phases illustrates the mechanism of sensing. The development of conductivity can monitor the degradation of samples. Actually, when the morphology of the samples changes during degradation, CNTs networks are modified, resulting in the variation in the conductivity of the nanocomposites. Accordingly, the evolution of conductivity during the degradation period develops an intelligent bio-system that senses the biodegradation. The extent of conductivity demonstrates the degradation level of the sample. Since large variations in conductivity can be easily detected, the samples containing high amounts of PEO and CNTs show strong sensing. On the other hand, the samples including low concentrations of PEO and CNTs cause insignificant changes in the conductivity, and thus they are not proper for degradation sensors. Therefore, PLA60/PEO40/CNT2 sample displays the highest sensing because it includes the maximum amounts of PEO and CNTs among the prepared samples.

3.3. Morphological observations Fig. 4 shows the SEM images of the samples containing 60 wt% PLA before degradation. The morphology of the blend shows the dispersion of PEO droplets in the continuous PLA phase. As a result, a matrix-droplet structure forms in the PLA60/PEO40 blend. The light particles in the PLA matrix are PEO because the wetting coefficient shows the dispersion of PEO particles in the continuous PLA. The addition of 1 wt% CNTs to this blend results in PEO islands in the continuous PLA, which is a matrix-droplet morphology. CNTs change the size and dispersion of PEO particles in the continuous matrix. In the nanocomposite, the size of PEO particles decreases and their dispersion in the PLA phase improves. The light strips demonstrate the CNTs in the nanocomposites. The content and size of the strips confirm this claim. In addition, the main difference between the SEM images of blends and nanocomposites are the light strips implying the CNTs in nanocomposites. As observed, CNTs are well dispersed in the polymer matrix indicating the strong interfacial interaction between the polymer and CNTs. The PLA60/PEO40/CNT2 sample also shows the formation of PEO droplets in the PLA phase. More CNTs reduce the size of PEO particles and homogenize their dispersion in the PLA phase. In addition, the distribution of CNTs in the PLA60/PEO40/CNT2 is regular confirming the formation of suitable nanoscale morphology in this sample. Interfacial energy minimization dictates the location of nanofillers in thermoplastic polymer blends, which expresses the strength of the driving force that arranges the nanoparticles in

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Fig. 3. The conductivity of nanocomposites containing a) 60, b) 75 and c) 90 wt % PLA at different degradation time. The errors bars indicate the standard deviation in the mean.

Fig. 4. SEM images of the samples containing 60 wt % PLA: a) PLA60/PEO40 blend, b) PLA60/PEO40/CNT1 and c) PLA60/PEO40/CNT2 before degradation.

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Fig. 5. SEM images of a) PLA75/PEO25 blend, b) PLA75/PEO25/CNT1 and c) PLA75/PEO25/CNT2 before degradation.

Table 1 The total surface energy of components and their dispersive and polar parts [58,59]. Materials

␥ (mN/m)

␥d (mN/m)

␥p (mN/m)

PLA PEO MWCNT

35.5 42.9 45.3

18.1 30.9 18.4

17.4 12 26.9

a particular phase. The wetting coefficient can be suggested as a simple description for this thermodynamic affinity [52] as: wa =

 PLA−MWCNT − PEO−MWCNT PLA−PEO

(5)

where “␥PLA-MWCNT ”, “␥PEO-MWCNT ” and “␥PLA-PEO ” are the interfacial energies between the components. At wa < -1, CNTs prefer the PLA phase, while wa > 1 displays the localization of CNTs in the PEO phase. Furthermore, wa values between -1 and 1 predict that CNTs are localized at the interphase between two polymers. The interphase in nanocomposites is established as a third phase due to the outstanding surface area of nanoparticles [53–56]. The interfacial energy between components can be calculated by a geometric-mean equation [57] as:



 12 = 1 + 2 − 2

 1d +



p

p

1 2

(6)

where “␥1 ” and “␥2 ” are the surface energy of components and “d” and “p” superscripts denote the dispersive and polar parts of surface energy, respectively. Table 1 shows the total surface energy as well as the dispersive and polar parts of the surface energy for PLA, PEO and MWCNTs. Eq. 6 calculates “␥PLA-MWCNT ”, “␥PEO-MWCNT ” and “␥PLA-PEO ” as 1.1, 4.66 and 2.2 mN/m, respectively. When these levels are substituted into Eq. 5, the wetting coefficient is calculated

as -1.62 demonstrating that the CNTs thermodynamically prefer the PLA phase for localization. Fig. 5 exhibits the SEM images of the prepared samples containing 75 wt% PLA before degradation. The blend system shows the formation of PEO islands in the PLA phase. Therefore, PEO particles are dispersed in the continuous PLA phase. This image indicates that the variation of PEO content does not affect the matrix-droplet morphology of the prepared blends. When 1 wt% CNTs is added to a PLA75/PEO25 blend, the PEO islands separate and form the PEO particles in the PLA continuous matrix. Therefore, CNTs can intensify the matrix-droplet structure of the PLA75/PEO25 blend. Also, the fine dispersion of CNTs in the PLA phase is clear in the PLA75/PEO25/CNT1 sample. Generally, an immiscibility between PLA and PEO phases is observed in the samples containing 60 and 75 wt % PLA, while the samples containing 90 wt % PLA and 10 wt % PEO are miscible (homogenous) [60]. The size of the light strips does not exceed 100 nm confirming the formation of nano-structures in the samples. The addition of further CNTs to this polymer blend causes the spherical and small PEO particles in the continuous PLA matrix. Generally, CNTs intensify the matrix-droplet morphology in the prepared nanocomposites and diminish the size of PEO particles. In fact, CNTs are localized in the PLA matrix, which affect the viscoelastic properties of the samples and change the formation/breaking of droplets during the fabrication process. Therefore, a large number of CNTs cause the small droplets and uniform dispersion of PEO in the samples. SEM pictures of the samples containing 60 wt% PLA after hydrolytic degradation in PBS solution for 4 weeks are shown in Fig. 6. The big particles of PEO are clear in the degraded blend. We also observe that the dispersion of PEO in the PLA phase is not homogenous. We conclude that the PEO phase partially degrades after 4 weeks and some PEO particles are still available in the

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Fig. 6. SEM images of a) PLA60/PEO40, b) PLA60/PEO40/CNT1 and c) PLA60/PEO40/CNT2 samples after degradation in PBS solution for 4 weeks.

matrix. As a result, the degradation of this blend produces dissimilar PEO droplets in the sample. The structure of the PLA60/PEO40/CNT1 sample is porous due to the degradation of PEO particles in a continuous matrix. In fact, the dispersed phase degrades causing a porous structure in the sample. The size of all pores is below 1 ␮m indicating that CNTs decrease the size of PEO droplets and unify the degraded regions. In addition, the content of remaining PEO droplets after degradation decreases due to the catalytic role of CNTs in the polymer degradation. The structure of PLA60/PEO40/CNT2 after degradation includes more small pores. The available PEO particles are very small and their dispersion in the PLA phase is homogenous. The small and uniform pores reveal the desirable role of CNTs in the degradation of PEO particles. The morphologies before and after degradation demonstrate that the hydrolytic degradation cannot change the matrix-droplet structure of samples. SEM images of the samples containing 75 wt% PLA after the degradation period are depicted in Fig. 7. The degraded blend is full of pores indicating the degraded PEO droplets. Also, some large PEO particles are not destroyed during the degradation period. However, the pores and the PEO particles are not uniformly dispersed in the sample. The PLA75/PEO25/CNT1 sample displays the islands of PEO, which are separated due to the presence of CNTs. It seems that some island areas degrade and others show no changes in the matrix. The samples with 2 wt% CNTs also exhibit the islands of PEO, but the islands are separated and their sizes are small. The high degradation of PEO particles caused by CNTs diminishes the size of PEO droplets. The degradation produces the PEO islands in the PLA matrix because the structures of PLA75/PEO25/CNT1 and PLA75/PEO25/CNT2 samples before degradation contain separated PEO particles in the continuous phase. We conclude that the degradation of PEO droplets and PLA matrix after 4 weeks causes the

island structures in these nanocomposites. The water from the PBS solution diffuses through the PLA to reach the PEO and hydrolyses the PEO, as the diffusion of ions in polymers is generally difficult. 3.4. Analysis of chemical bonds Fig. 8 (a, b) displays FTIR diagrams of prepared samples before degradation. The main peaks at 1080 and 1180 cm−1 show the stretching bonds of C–O in both PLA and PEO. In addition, the small peaks at 1350 and 1450 cm−1 indicate the bending bonds of C–H in PLA and PEO polymers. However, the main peak at a wavenumber of 1746 cm−1 reveals the stretching bond of C O in PLA. Additionally, the peak at 2900 cm−1 indicates C–H stretching bonds, which is very stronger in PEO than PLA. In fact, the PLA peak at about 2900 cm-1 is very weaker than other peaks, but PEO shows a very strong peak in this range. As a result, the peaks at 1746 cm−1 (C O) and 2900 cm−1 (C–H) can be considered as the determinate peaks for PLA and PEO phases, respectively. The prepared samples including blends and nanocomposites illustrate the mentioned peaks due to the presence of PLA and PEO polymers. The peaks attributed to C–O at 1080 cm−1 show some variations after the addition of CNTs, which is due to CNTs and the interactions between polymers and nanoparticles [15,61]. The intensity of the C O peak increases with the addition of PLA in the samples, while the peak for the C–H bonds becomes weaker due to the variations in the PLA and PEO fractions. Fig. 8 (c, d) shows FTIR graphs of samples after hydrolytic degradation for 4 weeks. The intensities of most peaks for PLA and PEO are smaller than those of before degradation because the degradation deteriorates the number of chemical bonds in both polymers. The peaks height ratios at 1080, 1746 and 2900 cm−1 are determined using Eqs. 3 and 4 to find the comparable results without

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Fig. 7. SEM pictures of a) PLA75/PEO25 blend, b) PLA75/PEO25/CNT1 and c) PLA75/PEO25/CNT2 samples after hydrolytic degradation for 4 weeks.

Fig. 8. FTIR diagrams of prepared samples containing a) 60 and b) 75 wt % PLA before degradation and c) 60 and d) 75 wt % PLA after degradation in PBS solution for 4 weeks.

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Table 2 FTIR peaks height ratios for the samples before and after degradation. Before degradation

Samples

PLA60/PEO40 PLA60/PEO40/CNT1 PLA60/PEO40/CNT2 PLA75/PEO25 PLA75/PEO25/CNT1 PLA75/PEO25/CNT2

After degradation

1080 cm−1

1746 cm−1

2900 cm−1

1080 cm−1

1746 cm−1

2900 cm−1

4.9 4.1 4.3 4.5 3.6 3.8

6.1 5.5 5.2 6.7 5.9 5.6

1.4 1.3 1.6 1.3 1.1 1.2

2.3 1.5 1.1 2.8 2.2 1.8

5.6 4.5 4.3 6.1 5.2 5.1

0.5 0.4 0.2 0.7 0.6 0.5

Fig. 9. WAXD patterns of the samples before degradation: a) 60, b) 75 and c) 90 wt % PLA and after degradation: d) 60, e) 75 and f) 90 wt % PLA.

experimental effect. Table 2 displays the peaks height ratios for the samples before and after degradation. The highest and the smallest peaks height ratios are observed for C O (1746 cm−1 ) and C–H (2900 cm−1 ) bonds, respectively. As indicated, the interactions between polymers and CNTs cause some variations in the peaks

height ratios for similar bonds in the blends and nanocomposites. All peaks height ratios decrease after degradation demonstrating that the degradation removes the chemical bonds of both PLA and PEO. The least variations of peaks height ratio are observed for C O bond in PLA at 1746 cm−1 because the degradation slightly changes

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the structure of PLA in the samples. However, the peaks height ratios for C–H bonds in PEO (2900 cm−1 ) suffer much reduction after degradation due to the high degradation of PEO phase in the samples. The degraded nanocomposites demonstrate the smaller peaks height ratios compared to the degraded blends confirming that CNTs accelerate the degradation of polymers in PBS solution. The PLA60/PEO40/CNT2 sample shows the lowest peaks height ratios after degradation because the high amounts of PEO and CNTs induce the high degradation of chemical bonds. On the other hand, PLA75/PEO25 blend depicts the highest peaks height ratios after degradation because a low number of PEO particles in the absence of CNTs prevent the degradation of chemical bonds. These findings agree with the mentioned results of degradation fraction for the prepared blends and nanocomposites (Fig. 1). 3.5. Crystalline structures of polymers Fig. 9 (a, b, c) reveals the WAXD patterns of the prepared samples before degradation. Three main peaks at 16.5◦ , 19◦ and 23.1◦ are clear in the samples. The peaks at 16.5◦ and 19◦ show the diffraction of (110)/(200) and (203) crystal planes of ␣-PLA, respectively [20,48], while the crystals of PEO were evident at 23.1◦ [48,62]. The stronger peaks of PLA crystals compared to PEO express the high level of crystallinity in the PLA matrix. The addition of 1 wt% CNTs to the PLA60/PEO40 blend increases the intensity of all peaks because the nucleating efficiency of the nanoparticles promotes the crystallinity of polymers [63,64]. However, a further increase in CNTs deteriorates the main peaks indicating the low amount of crystals in the nanocomposite. It seems that a large number of CNTs limit the movement of polymer chains into crystalline lamellae preventing the formation of complete crystals. Moreover, the peak at 23.1◦ for PEO crystals negligibly changes proving the localization and efficiency of CNTs in the PLA phase. The WAXD diagrams of samples after degradation are observed in Fig. 9 (d, e, f). All samples show that the main peaks for all crystals change indicating that the degradation changes the crystalline regions of both PLA and PEO. Additionally, all samples illustrate very weak peaks at 23.1◦ signifying that the crystallinity of PEO deteriorates after degradation. In fact, both amorphous and crystalline regions of PEO undergo the hydrolytic degradation in PBS solution. It is also clear that further addition of CNTs into nanocomposites does not change the crystalline peaks of PEO, while PLA shows different peaks as the CNTs concentration increases. As a result, CNTs mainly control the crystallinity of PLA in the nanocomposites confirming the previous expressions for localization of CNTs in the PLA matrix. In PLA60/PEO40 and PLA60/PEO40/CNT1 samples, the PLA crystalline peaks become weaker after degradation demonstrating that PBS solution attacks the amorphous and crystalline regions of these samples. However, the PLA60/PEO40/CNT2 sample shows larger peaks after degradation indicating that the amorphous regions undergo degradation and the degraded PLA chains form suitable crystals. 4. Conclusions The hydrolytic degradation and sensing behavior of PLA/PEO/CNTs nanocomposites in PBS solution were investigated. Additionally, the variations in polymer molecular weight, morphology, chemical bonds and the crystalline structure of polymers due to degradation were analyzed. The samples rapidly degraded during the first week, but the degradation fraction and rate insignificantly changed after 1 week. CNTs promoted the degradation of polymers because CNTs played a catalytic role in the degradation. The molecular weight of PEO mainly decreased during degradation, but PLA showed negligible variation in molec-

ular weight at different times. The conductivity of all samples improved with degradation time and the samples containing the high fractions of PEO and CNTs showed high sensing. At least twelve SEM images displayed a matrix-droplet structure in the samples, where more CNTs decreased the size of PEO particles and improved their dispersion in the PLA phase. The degradation negligibly changed the morphology of the samples, but the size and number of pores and PEO particles after degradation depended on PEO and CNTs fractions. The degradation weakened the intensities of FTIR peaks and reduced the peaks height ratios for PLA and PEO, but the variations of peaks height ratios at 1746 cm−1 were unimportant demonstrating that the PLA phase insignificantly degraded in PBS solution. In comparison, the peaks height ratios at 2900 cm−1 greatly decreased after degradation illustrating the high degradation of PEO in PBS solution. All samples showed very weak WAXD peaks at 23.1◦ after degradation signifying that the crystallinity of PEO mainly deteriorated by degradation. CNTs did not change the crystalline peak of PEO, while PLA displayed different peaks as CNTs concentration increased indicating that CNTs largely controlled the crystallinity of the PLA matrix.

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Biographies

Yasser Zare received his Ph.D from Department of Polymer Engineering and Color Technology at Amirkabir University of Technology (Tehran, Iran). He studied the degradation and nanobiosensing behavior of PLA/PEO/CNT nanocomposites in Ph.D thesis. He has joined to Department of Mechanical Engineering at Kyung Hee University (Republic of Korea) since 2016. He is interested in research and development of polymer nanocomposites from experimental and theoretical points of view. He has focused on the nanobiosensing behavior of polymer nanocomposites.

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Hamid Garmabi is Professor of Department of Polymer Engineering and Color Technology at Amirkabir University of Technology (Tehran, Iran) from 1997. He studied chemical engineering at Sharif University of Technology and then received his M.Sc. degree in 1991 from Amirkabir University of Technology. He received his Ph.D. from Chemical Engineering Department of McGill University (Canada) in 1996. His research interests include morphology control of polymer blends and nanocomposites (processing, rheology, characterization and prediction of physical/mechanical properties) with special attention on biomaterials.

Kyong Yop Rhee is Professor of mechanical engineering at Kyung Hee University (South Korea) from 1999. He earned his Bachelor (1984) and Master (1986) degrees at Department of Mechanical Engineering of Seoul National University (South Korea). He received his Ph.D. from mechanical engineering at Georgia Institute of Technology in 1991. His main research interests are nanocomposites, surface treatment, fracture, fatigue and sintering.